The present invention relates to a circuit for converting an analog video signal into binary signals in an electronic black board, a telecopier and the like, and particularly to an improvement in setting the slice levels.
In electronic black boards, it is customary to use a fluorescent lamp FL as shown in FIG. 1 to illuminate a movable screen MS on which characters, symbols and pictures are written and light reflected from the movable screen MS is focused by a lens LZ onto a line image sensor such as a linear CCD (charge-coupled device) by which the optical signal is converted into an electric signal, called a video signal. The video signal is compared with a slice level or reference level to produce a series of binary signals indicating black or white of respective pixels.
The light intensity of the fluorescent lamp FL is not uniform along its length, and it varies depending on the temperature of the tube. This means that the light intensity distribution varies with time after the fluorescent lamp is turned on. For instance when the fluorescent lamp has just been turned on the intensity is generally low but the upper part of the tube which is heated more quickly has a relatively high intensity, as shown in FIG. 2A. When the entire tube is fully heated, the intensity is the highest at the center and is decreased towards both ends, as shown in FIG. 2B. Moreover, the lens has a property by which the light having passed the lens has the intensity which is increased toward the optical axis.
As a result, the white level of the video signal varies with time and along the length of each scan.
To cope with the nonuniform distribution of the light intensity along the length of each scan, a shading plate having a greater shading rate toward the center has been used. But positioning the shading plate demands extreme accuracy and is time consuming. Moreover it does not provide a measure against the change of the light intensity distribution with temperature.
Another solution is to electronically vary the reference level in conformity with the white level variation. This can be done by setting the reference level based on the signal levels obtained from the marginal area of the screen, before the processing of the object image signal (effective video signal) starts. Here, "marginal area" means the area on the screen adjacent to the edge of each "cut" of the images on the screen. By 37 cut" is meant that portion of the screen on which optically readable information to be processed is present, as shown in FIG. 3.
Usually nothing is written on the marginal area. But the user may write on the marginal area and there can be some smear on the marginal area, so that the device is expected to operate properly even if there are some writings or smears in the marginal area.
A prior art arrangement determines the white level for each dot of the line sensor based on data from several lines (scans): the highest of the levels of video signals of the same dot of the sensor obtained through the several lines is taken as the white level and the reference level is set based on the white level. For instance, the reference level is given by multiplying a coefficient, e.g., 0.7 with the white level.
But this prior art arrangement fails to find a proper white level when a black line (a written one or a smear) extends horizontally (normal to the length of the line sensor) over several lines.